1. Field of the Invention
The present invention relates to an air pump, and more particularly, to an air pump for use in combination with a household aquarium in which fishes and aquatic plants are reared for fancy or ornamental purposes.
2. Description of the Prior Art
So far there have been many types of air pumps for such purposes, among which is one disclosed in Japanese Utility Model Publication No. 47(1972)-26002. This prior art pump is provided with a diaphragm fixed to a vibrator, which is integral with a permanent magnet. The permanet magnet is placed in a magnetic field. By switching on the a.c. power, the permanent magnet is excited, thereby causing the diaphragm to vibrate through the movement of the vibrator. In this case, the vibrator is fixed to the casing of the pump, thereby unavoidably causing the casing to vibrate. This is the cause of noises. In addition, a resisting force exerts on the movement of the vibrator at the joint thereof to the casing, which leads to the energy loss. Furthermore, it is difficult to arrange the components linearly, and especially because of using a U-shape iron core the whole size becomes large. For such uses pumps should be as small as possible.
To prevent the energy loss and the harsh noises, many proposals have been made. Japanese Utility Model Publication (unexamined) No. 53(1978)-140906 discloses one of the proposals. This prior art pump is designed to vibrate the diaphragm directly by means of a permanent magent so as to prevent the energy loss. To this end, the pump is provided with a driving shaft integral with a permanent magnet, the driving shaft being connected to a diaphragm. There are provided two electromagnets opposedly to the permanent magnet. It is true that this arrangement has solved the problems of noise and energy loss, but because of the provision of the two electromagnets, which are essential for balancing the driving shaft, the size of the pump becomes large.
Another solutions have been proposed by Japanese Utility Model Publication Nos. 48(1973)-36247 and 47(1972)-26404. The pumps disclosed in these two specifications are provided with diaphragms fixed to vibrators, which have permanent magnets located opposedly to the iron cores of electromagnets. These pumps also employ U-shape iron cores, which results in an increased size as a whole. In addition, the noise resulting from the vibration of the vibrator is very high.